Invert the scroll direction for wheel events on internal windows

Summary:
For whatever reason Qt scrolled the wrong way on internal windows. As a
new QWheelEvent is constructed anyway, just multiply by -1 to get the
direction fixed.

BUG: 371999
FIXED-IN: 5.8.4

Test Plan: Tried debug console in nested setup, scrolled correctly now.

Reviewers: #kwin, #plasma_on_wayland, broulik

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D3247